The present invention relates generally to diagnostic imaging and, more particularly, to a CT detector having an optical mask layer to reduce cross-talk between scintillators and neighboring photodiodes.
Typically, in computed tomography (CT) imaging systems, an x-ray source emits a fan-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis which ultimately produces an image.
Generally, the x-ray source and the detector array are rotated about the gantry within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator for converting x-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom.
Typically, each scintillator of a scintillator array converts x-rays to light energy. Each scintillator discharges light energy to a photodiode adjacent thereto. Each photodiode detects the light energy and generates a corresponding electrical signal. The outputs of the photodiodes are then transmitted to the data processing system for image reconstruction.
Generally, the photodiode is used to convert light signal or energy received to an electric current. Typically, the amount or value of the electric current generated is linearly proportional to the amount of light energy or signal detected. In this regard, for efficient and effective image reconstruction, it is imperative that the x-rays received by a scintillator, light emitted by the scintillator, and the light detected by the photodiode be localized. That is, the quality of the output of a photodiode may be compromised if there is a cross-communication between neighboring detector cells. This cross-communication is generally referred to as “cross-talk”.
“Cross-talk” between detector cells of a CT detector occurs when data or signal is transferred between neighboring detector cells. Generally, cross-talk is sought to be reduced as cross-talk leads to artifact presence in the final reconstructed CT image and contributes to poor spatial resolution. Typically, four different types of cross-talk may result within a single CT detector. X-ray cross-talk may occur due to x-ray scattering between scintillator cells. Optical reflector cross-talk occurs by transmission of light through the reflectors that surround the scintillators. Known CT detectors utilize a contiguous optical coupling layer(s), typically epoxy, to secure the scintillator array to the photodiode array. This coupling layer produces an optical coupling cross-talk. This is due to light which is trapped and passes through the optical coupling layer into the region above an adjacent diode, where it is finally absorbed in the diode and converted to electrical signal. A fourth type of cross-talk is diffusion cross-talk. It is a combination of electrical and optical. This is due to optical photons which are typically generated close to the boundary between diode regions. The optical generated carriers diffuse within the field free regions of the diode and some are collected by adjacent diodes producing cross-talk. Both optical coupling cross-talk and diffusion cross-talk as defined above will be described as optical transference or optical transference cross-talk.
Cross-talk and especially cross-talk variation is a significant source of artifacts in CT imagers. To reduce cross-talk variation (in any form) between detector cells, the CT detector is manufactured to extremely tight tolerance so that high quality and artifact-free CT images may be reconstructed. Many of the cross-talk mechanism are very sensitive to small variations in detector dimensions and other properties. The misalignment of the scintillator array to the photodiode array drives the non-uniformity of cross-talk level from one cell to its neighbors. In order to reduce the variation of crosstalk from one cell to another, either a better alignment of diode to scintillator or a non-sensitive design is required.
One of the contributors of this cell-to-cell variation of cross-talk is the electrical cross-talk generated by diffusion of photo generated carriers between the individual photodiode elements. The lateral diffusion of photo generated carriers can be thought of as an increase in the photoactive area of the photodiode collection junction. This lateral diffusion leads to a lateral cross-talk which occurs when some photocarriers diffuse out of the cell collection site in which they are generated and are collected by a neighboring cell. This effect is even more pronounced in back-illuminated diodes because the thickness of the diodes increases the diffusion length before collection. A back-illuminated diode is a diode array where the light is incident on the side of the diode array opposite from the diode junctions.
It would therefore be desirable to design a CT detector with reduced electrical cross-talk for improved image reconstruction.